This application claims priority to Italian Application Serial Number 2002A000941, filed Sep. 11, 2002.
1. Field of the Invention
The present invention relates to charge pump circuits. More particularly, the present invention relates to a digital-to-analog-converter (DAC) based voltage regulator system that can regulate the output of charge pump circuits in a non-volatile memory for supplying various voltages utilized in the memory circuit.
2. The State of the Art
Non-volatile memories require a broad range of voltages that are applied to the word lines coupled to the gates of the memory cell transistors. A plurality of voltage regulators provide the different appropriate voltages for each operation of the device. The output voltage of each voltage regulator is fixed and the re is no way to change the output voltages of the regulator devices.
A non-volatile memory requires at least a voltage regulator for supplying a voltage to the memory cell gates for verifying a programmed cell and an erased cell, and a voltage regulator for supplying a programming voltage to the memory cell gates for programming memory cells. In some non-volatile memory devices the reliability of the cell requires that the program voltage not be constant but rather that the voltage is ramped. Additional analog circuits need to be provided for this purpose.
The present invention provides a DAC-based voltage regulator for supplying the necessary regulated voltages to a non-volatile memory device. The voltage regulation is based on a current comparison between a reference current and a current signal obtained from the high-voltage output of the charge pump circuit. The N bits provided to the DAC generate 2N current-reference levels that are sunk from a virtual ground. A voltage to current converter generates a current signal from the high-voltage output of the charge pump that is supplied to virtual ground. The current-error signal that comes from the comparison in the virtual ground is amplified and voltage converted by a transconductance amplifier. A second amplifier receives the output signal from the transconductance amplifier and outputs a CMOS-compatible voltage level that enables or discharges the charge pump in order to obtain the selected output voltage value. Thus, the charge pump can be regulated to one of 2N values according to the input presented to the DAC.
The voltage regulator of the present invention is capable of supplying a wide range of output voltages to bias the word lines in a non-volatile memory device. N input signals are provided in order to select the appropriate output voltage for each operation of the non-volatile memory. Because of this feature, a voltage ramp can easily be generated by providing the sequence of the values of the ramp.